Re: [PATCH v2 3/4] tools headers UAPI: add cpu_relax() implementation for x86 and arm64

On 2021/7/22 4:53, David Laight wrote:

From: Yunsheng Lin

quoted

Sent: 20 July 2021 03:22

As x86 and arm64 is the two available systems that I can build
and test the cpu_relax() implementation, so only add cpu_relax()
implementation for x86 and arm64, other arches can be added easily
when needed.

...

quoted

+#if defined(__i386__) || defined(__x86_64__)
+/* REP NOP (PAUSE) is a good thing to insert into busy-wait loops. */
+static __always_inline void rep_nop(void)
+{
+	asm volatile("rep; nop" ::: "memory");
+}

Beware, Intel increased the stall for 'rep nop' in some recent
cpu to IIRC about 200 cycles.

They even document that this might have a detrimental effect.
It is basically far too long for the sort of thing it makes
sense to busy-wait for.

Thanks for the info:)
I will be beware of that when playing with  'rep nop' in newer
x86 cpu.

.